How big is the property that her mothers house is on ?
How much soft soil is there within a few hundred yard radius of the home ? Is there a crawl space under the house ?
Is there a patch of woods or overgrown vegetation nearby ?
She disappeared in the evening.....her mom was sick in bed.......darkness was coming on.........perfect time to dig and hide something.
Who knows if the mother was in and out of sleep and for how many hours, especially if taking flu meds.
Search the obvious ! Crawl under the house, bring in cadaver dogs, look for recently disturbed grass or vegetation in the yard.
If this is a criminal act..........then she is still right there under their noses.